Kristály Cave is a small but extraordinary karst cave hidden in the wall of a disused quarry in the Pilis Mountains. Its name — meaning 'Crystal Cave' — is well earned: the walls inside are lined with striking calcite crystals and 'pea-stone' formations that have survived largely untouched thanks to the cave's extremely narrow, hard-to-reach passages.

This is a classic thermal-water cave, formed in Upper Triassic Dachstein limestone, consisting of four small chambers connected by tight crawlways. Strong air currents can be felt inside, and in winter the air is noticeably warm. Because of its fragile, rare mineral formations and cramped passages, entry requires basic caving equipment and is only permitted with authorization — this is a closed cave, not open to casual visitors.

How to get there

• Green-marked trail near Nagy-Kevély — leads to the disused quarry marked on tourist maps, from where the cave entrance is easily visible in the quarry wall, about 11 m above its floor
• Zöld-barlang (Green Cave) lies nearby, roughly 100 m away

Practical information

• The cave holds special protected status and is closed — visits are only possible with permission from the Duna-Ipoly National Park Directorate
• Basic caving equipment is required
• The entrance is located in the wall of a former quarry, reachable from the green trail

Interestingly, the entire wall surface of the cave was once completely covered in calcite crystals — many of these delicate formations survive today precisely because the tight passages kept casual visitors away.